Thank you Oleg. I have deleted the logs, activated Calculate changelog.
The display is now clean but since there is nothing to display I launched a new sitemap.
Strangely, although there is no structure changed on the site, now the crawler stops on the first page as if there were no links; but the links are still there like before. I have tried on both servers (the dev and the prod servers) and it's the same. You can see in the screen shot attached from History: 3,909 pages crawled earlier (= normal) and only 1 page now (incorrect). I have no idea how or why. I've reviewed the crawl settings and aside from the Calc I just activated, they are absolutely the same as before.